CHAP. VIII. DIAHELIOTEOPISM. 443 



any service to the plant, so as to have been specially 

 acquired. It must be the result of some periodical 

 change in the conditions to which they are subjected, 

 and there can hardly be a doubt that this is the daily 

 alternations of light and darkness. De Yries states in 

 the paper before referred to, that most petioles and 

 midribs are apogeotropic ;* and apogeotropism would 

 account for the above rising movement, which is com- 

 mon to so many widely distinct species, if we suppose it 

 to be conquered by diaheliotropism during the middle 

 of the day, as long as it is of importance to the plant 

 that its cotyledons and leaves should be fully exposed 

 to the light. The exact hour in the afternoon at which 

 they begin to bend slightly upwards, and the extent of 

 the movement, will depend on their degree of sen- 

 sitiveness to gravitation and on their power of resist- 

 ing its -action during the middle of the day, as well as 

 on the amplitude of their ordinary circumnutating 

 movements ; and as these qualities differ much in dif- 

 ferent species, we might expect that the hour in the 

 afternoon at which they begin to rise would differ 

 much in different species, as is the case. Some other 

 agency, however, besides apogeotropism, must come 

 into play, either directly or indirectly, in this upward 

 movement. Thus a young bean (Viciafaba), growing 

 in a small pot, was placed in front of a window in a 

 klinostat ; and at night the leaves rose a little, although 



* According to Frank ('Die organs have been long kept in the 

 nat. Wagerechte Kichtung von dark, the amount of water and of 

 Ptfanzentheilen,' 1870, p. 46) the mineral matter which they con- 

 root-leaves of many plants, kept tain is so much altered, and their 

 in darkness, rise up and even be- regular growth is so much dis- 

 come vertical ; and so it is in some turbed, that it is perhaps rash to 

 cases with shoots. (See Eauvven- infer from their movements what 

 hoff, 'Archives Neerlandaises,' would occur under normal con- 

 torn, xii. p. 32.) These movements ditions. (See Godlewski, *Bot. 

 indicate apogeotropism ; but when Zeitung,' Feb. 14th, 1879.) 



